The present invention relates to mass spectrometry, and more particularly to field ionization and field desorption mass spectrometry.
Field ionization (FI) and field desorption (FD) mass spectrometry are increasingly important techniques for studying thermally unstable organic molecules, especially those which are biologically important. With either technique, positive ions of a meterial being analyzed are generated by exposing the material to a high magnitude electric field (of the order of 10.sup.7 -10.sup.8 V/cm). Of the various types of field anodes or emitters which have been used for producing such fields, including single sharp metal points, sharp metal edges, and activated thin wires, the latter have proved most suitable.
"Activation," as the term is used herein, is the production of a substantial number of semiconducting microneedles on the surface of a field anode to provide local field enhancement. The microneedles are grown on the emitter by field polymerization or field pyrolysis of an organic compound, usually benzonitrile. For FI work, densely activated, short needle (.about.1 um) emitters are considered preferable, but for FD work, emitters having much longer needles (10-25 um) are used.
Several methods for activating thin metal wires are known. Room temperature activation has been described, for example, by Beckey et al. in Messtechnik, 79, (1971) 196-20l. Activation at considerably higher temperatures (1200.degree.C.) has been detailed by Beckey et al. in J. Phys. E: Scientific Instruments, 6, (1973) 1040-44. High temperature activation is considered superior, since the microneedles produced at high temperature have a higher thermal stability, much higher resistance to chemical attack, and a reduced tendency to yield to high electric field stresses.
Attempts to produce activated thin wire emitters using prior art methods have met with many difficulties, particularly in initiating the desired dense, uniform needle growth. In many instances, activation did not proceed at all, and when microneedles were produced, they were sparsely distributed in tufts over the surface of the wire. Good emitters are essential to the operation of an FI/FD ion source, and the development of FD mass spectrometry in particular has been inseparably linked to improvements in the technique for making field anodes.
There is a need, therefore, for an improved method for reliably producing activated thin wire emitters for field ionization and field desorption mass spectrometry.
A general object of the present invention is to provide a method which meets the above-identified need in a practical and satisfactory manner.
A more specific object of the invention is to provide a method for reproducibly growing microneedles on thin wire field anodes.
A related object of the invention is to provide improved, activated thin wire emitters for FI/FD mass spectrometry.